The "SKSE 1.6.342" version (more accurately known as version of the SKSE64 build) marks one of the most significant—and frustrating—chapters in The Elder Scrolls V: Skyrim Special Edition modding history.
For a long time, 1.6.342 was the "stable" version of the Anniversary Edition. Many modders transitioned their work to this build before Bethesda released subsequent updates (like 1.6.640 and the more recent 1.6.1130+). skse 16342
If you have disabled automatic updates on Steam and your game version is currently 1.6.342, you use SKSE version 2.1.2. Using a newer or older version of SKSE will cause the game to crash on startup or display the "Version Mismatch" error. How to Install SKSE for 1.6.342 The "SKSE 1
Copy the .dll and .exe files into your main Skyrim directory (where the game's .exe is located), not the Data folder. If you have disabled automatic updates on Steam
If your mod list is currently stable on 1.6.342, the general consensus in the modding community is: Are you trying to fix a crash on startup, or